1. Field of the Invention
The present invention relates to fiber optic gyroscopes. More particularly, this invention pertains to a sensor coil design that addresses the effects of temperature-change-induced optical length asymmetry upon bias stability.
2. Description of the Prior Art
An interferometric fiber optic gyroscope comprises the following main components: (1) a light source, (2) two beamsplitters (fiber optic directional coupler and/or integrated-optics Y-junctions) to satisfy the requirement of a "minimum reciprocal configuration" (S. Ezekiel and M. J. Arditty, Fiber Optic Rotation Sensors New York, Springer-Verlag p. 2-26 1982), (3) a fiber sensing optic coil made of either polarization maintaining (PM) fiber or low-birefringence fiber, (4) a polarizer (and sometimes one or more depolarizers), and (5) a detector. Light from the light source is split by the loop beamsplitter into copropagating and counterpropagating waves travelling in the sensing coil. The associated electronics measures the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams is proportional to the rate of rotation of the platform to which the instrument is fixed, due to the well-known Sagnac effect.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. In general, such factors are both time-varying and unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
One approach to reducing bias sensitivities arising from environmental factors has involved the use of various symmetric coil winding configurations. In such coils, the windings are arranged so that the geometrical center of the coil is located at the innermost layer while the two ends of the coil are located at the outermost layers.
N. Frigo has proposed the use of particular winding patterns to compensate for non-reciprocities in "Compensation of Linear Sources of Non-Reciprocity in Sagnac Interferometers" , Fiber Optics and Laser Sensors I, Proc. SPIE Vol. 412 p. 268 (1983). Furthermore, U.S. Pat. No. 4,793,708 of Bednarz entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil formed by dualpole or quadrupole winding. The coils described in that patent exhibit enhanced performance over the conventional helix-type winding.
U.S. Pat. No. 4,856,900 of Ivancevic entitled "Quadrupole-Wound Fiber Optic Sensing Coil and Method of Manufacture Thereof" teaches an improved quadrupole-wound coil in which fiber pinching and microbends due to the presence of pop-up fiber segments adjacent the end flanges are overcome by replacing such pop-up segments with concentrically-wound walls of turns for climbing between connecting layers. Both of the aforementioned United States patents are the property of the assignee herein.
While appropriate coil winding techniques minimize some of the effects of some sources of bias error in the output of a fiber optic gyro, they do not reduce the bias completely.
A large fraction of gyro bias instability results from the so-called temperature-change-induced optical length asymmetry (TCIOLA) of the fiber of the wound coil with respect to its midpoint. The largest component of TCIOLA results from thermal stress within the wound coil which increases as the temperature departs from the polymer curing temperature. (Note: the coil is nominally stress-free at the polymer curing temperature.)
While the above-described winding techniques achieve some reduction in TCIOLA induced bias effects, by arranging the coil so that fiber segments at equal distances from the midpoint of the wound fiber lie adjacent one another, high accuracy applications, such as long-duration aircraft navigation, require additional reductions in this bias factor.